1. Field of the Invention
The present invention relates to an electrical feed and control circuit for a drive arrangement of the pump piston of a spray gun which comprises a coil and an oscillating armature, an input for connection to an alternating current network, an output leading to the oscillating armature coil, a half-wave suppression circuit selectively connectible by way of a transfer switch, and a phase control circuit which can be connected and disconnected by the transfer switch together with the half-wave suppression circuit, the phase control circuit being provided with a timing element including a fixed resistor and a capacitor.
2. Description of the Prior Art
In spray guns having a pump piston driven by an oscillating armature, the oscillating armature coil is essentially connected directly at the alternating current network, this leading the fact that the pump piston executes 6000 stroke/min. By way of a corresponding design of the overall arrangement, particularly of the oscillating armature, pump piston, pull-back spring, nozzle, etc, is that the atomization is very good for a prescribed, maximum paint throughput of the gun, i.e. the paint is finely atomized. However, a satisfactory fine atomization also occurs when the paint feed is throttled, the paint throughput is therefore reduced. Expressed in other terms, the degree of atomization remains essentially the same over a specific paint throughput range. However, a disadvantage with these known paint spray guns is that they must work with a high piston frequency of 6000 strokes/min even with a throttled paint throughput, i.e. must work at a piston frequency which leads to wear and to rather considerable noise. Therefore, the spray guns have appeared on the market in which the piston frequency can be selectively reduced from 6000 to 3000 stroke/min, i.e. for low amounts of paint throughput. This halving of the frequency occurs in such a manner that a feed and control circuit is provided between the network and the oscillating armature coil, the feed control circuit containing a half-wave suppression circuit which suppresses the half-waves of one polarity of the alternating current. However, satisfactory results have not been achieved with such an arrangement, this being attributable to the fact that the overall arrangement, of course, is optimized for operation of 6000 stroke/min and, therefore, with a halving of the overall amount of paint ejected given the same nozzle size, a considerable deterioration of the quality of atomization occurs. Moreover, the increase of peak and effective current necessarily occurring from the half-wave suppression leads to a considerable heating of the oscillating armature coil. In order to produce an improvement in this respect, a spray gun has already been disclosed in which, together with the transfer to operation having 3000 stroke/min, a phase-angle control of the remaining half-waves is carried out. In addition to the half-wave suppression circuit, the feed and control circuit therefore contains a timing element comprising fixed resistors and a capacitor. The phase-angle control during operation at 3000 stroke/min effects a reduction of the output of the device and therefore reduces the danger of overheating of the oscillating armature coil. A good atomization quality even given this mode with reduced frequency can be achieved, moreover, by a suitable dimensioning of the (fixed) size of the phase-angle control.
However, all described, known devices share the disadvantage that the operator has no possibility of significantly changing the degree of atomization, in particular independently of the amount ejected. In particular, practice is not such that a superfine atomization (high degree of atomization) is always desirable; on the contrary, there are often instances in which atomization in coarser particles or even a so-called wet stream is desirable; therefore, particularly when spraying with pesticides, a coarser atomization is preferable to a superfine atomization (fog-like cloud).